This invention relates to a method of making sheets, including thin films and relatively thick plates, of high optical quality of transparent plastics material. The invention also relates to apparatus suitable for carrying out this method.
There have already been obtained films or sheets of plastics material having a high optical quality by a process in which a solution or molten mass of the plastics material reaction mixture which produces the plastics material is fed in the liquid state by a suitable device, for example a linear slot, a brushing device or a flow head, onto heated drums or endless metallic bands having a surface which is optically smooth. The plastics material hardens in situ under the action of the heat and is then separated from the drum or band support in the form of an endless sheet or film.
In this known process it is difficult to avoid the polished metal surface being subject to mechanical deformation. The quality of the optically polished surface may also be affected by constant re-use of the band or drum. Consequently there is a risk that the state of the surface of the sheet of plastics material, which largely determines its optical quality, will deteriorate due to wear of the support according to its extent of previous use.